The present invention relates to a method and an apparatus for controlling a switched reluctance motor (hereinafter, referred to as an SR motor) and to a compressor that employs the SR motor, which is controlled by the controlling apparatus.
Compared to a conventional alternating motor, an SR motor is smaller and has a simpler structure, which contributes to a reduction in costs. The current or voltage of the SR motor needs to be controlled in accordance with the position of a rotor due to the principle of torque generation. Therefore, an optical or magnetic positioning sensor for the rotor is generally located on a motor axis of the SR motor. However, if the positioning sensor is located on the SR motor, the size of the system that is equipped with the SR motor is increased and the reliability is decreased. Also, the installation requirements of the system having the SR motor restrict the installation position. This spoils the advantages of the SR motor.
Therefore, a method and an apparatus for controlling the SR motor without providing a positioning sensor on the SR motor have been developed. In this method and apparatus, the SR motor is controlled by estimating the position of the rotor. FIG. 5 is a graph showing the magnetizing properties of the SR motor. FIG. 5 shows a one-to-one relationship between the magnetic flux linkage xcex and the coil current i for each rotor position xcex8 (the rotor position xcex8 is 90 degrees at an aligned position, the rotor position xcex8 is 45 degrees at a nonaligned position).
It is considered that the position of the rotor can be estimated based on the magnetizing properties. For example, The Transactions of the Institute of Electrical Engineers of Japan D (June, 2000 Publication No. 795-801) focuses on the fact that each rotor position has a one-to-one relationship with the coil current and the magnetic flux linkage of one phase. The above publication discloses a simplified equation of the magnetizing properties for estimating the position of the rotor and proposes an algorithm of position estimation based on the equation.
More specifically, the above publication discloses a simple equation, which describes the magnetic flux linkage xcex by the coil current i and the rotor position xcex8 based on the following basic equation (1), which describes the magnetic flux linkage xcex(t). Thus, the rotor position is estimated using the coil current i and the magnetic flux linkage xcex.
xcex(t)=∫{Vph(t)xe2x88x92Rxc2x7i(t)}dtxe2x80x83xe2x80x83(1) 
In the above equation, Vph(t) represents the applied voltage to the coil, R represents the coil resistance, and i(t) represents the coil current.
Japanese Laid-Open Patent Publication No. 2001-128477 discloses a controller in which an inverter supplies pulse voltage or current to a stator coil of each phase of an SR motor, thereby actuating the SR motor. The controller includes reference signal generating means, which generates a reference signal regardless of the position of a rotor of the SR motor, and rotational speed detector, which detects the rotational speed of the rotor without using the reference signal. Based on the generated reference signal and the detected rotational speed, the controller controls the inverter by inverter controlling means to set the phase of pulse voltage or current such that a forward torque is generated. The rotational speed detector also has a detector for detecting the magnetic flux linkage xcex.
However, even though a simple equation is used in the position estimating algorithm of the former method, the calculation for estimating the rotor position xcex8 is complicated. Thus, it is required to use a high-performance CPU or a digital signal processor (DSP), which increases the cost.
In the controller of the publication No. 2001-128477, in addition to the reference signal generating means, which generates a reference signal regardless of the position of the rotor of the SR motor, the rotational speed detector, which detects the rotational speed without using the reference signal, is required. This complicates the structure of the controller and increases the load on the CPU. As a result, a high-performance CPU is required.
Recently, a system (idling stop system), which automatically stops an engine when a vehicle temporarily stops at stoplights while traveling on urban loads and restarts an engine under a predetermined condition, has been developed to reduce the exhaust gas and economize on fuel. An actuator for such vehicle equipment is disclosed in, for example, Japanese Laid-Open Patent Publication No. 9-324668. The actuator actuates a compressor, which is located in a refrigeration circuit of a vehicular air-conditioner, by the power from an engine when the engine is running. When the engine is stopped, the compressor is actuated by a motor. The motor, which is mounted on the compressor of the vehicular air-conditioner, is actuated at a relatively low speed (less than or equal to 1000 rpm) and at a constant torque when it is actuated during idling stop of the engine. Thus, when SR motors are used for such motor, the rotational speed need not be detected accurately. Thus, it is only required that the phases be switched such that the motor is reliably driven by a forward torque.
Accordingly, it is an objective of the present invention to provide a method and an apparatus for controlling an SR motor that has a simple structure and reduces the load on a CPU and that is suitable for a case in which a motor is driven at a relatively low speed and at a constant torque. The present invention further provides a compressor that employs the SR motor that is driven by the above control method.
To achieve the above objective, the present invention provides a method for controlling a switched reluctance motor. The switched reluctance motor has a plurality of coils. Each of the coils cycles through a cycle of phases including an excited phase. The method includes: controlling an applied voltage to supply constant current to each of the coils of the switched reluctance motor in order of rotation; and determining a switching timing for switching the phases of the coils such that the switch to the exited phase occurs when the variation amount of the applied voltage to the coil having the exited phase is substantially zero.
The present invention also provides a controller located in a switched reluctance motor having a plurality of phases. The switched reluctance motor has a plurality of coils. Each of the coils cycles through a cycle of phases including an excited phase. The controller has a first detector, a second detector, an inverter and an inverter controlling device. The first detector detects the current supplied to the coils. The second detector detects the voltage applied to the coils. The inverter generates the current supplied to the coils. The inverter controlling device controls the inverter such that the current value is constant. The inverter controlling device controls the inverter to switch the excited phases when the applied voltage is less than or equal to a predetermined value that is determined based on the product of the resistance of the coil corresponding to the applied voltage and the current value detected by the first detector.
The present invention also provides a compressor including a switched reluctance motor having a plurality of phases. The switched reluctance motor has a plurality of coils. Each of the coils cycles through a cycle of phases including an excited phase. The switched reluctance motor includes a controller. The controller has a first detector, a second detector, an inverter and an inverter controlling device. The first detector detects the current supplied to the coils. The second detector detects the voltage applied to the coils. The inverter generates the current supplied to the coils. The inverter controlling device controls the inverter such that the current value is constant. The inverter controlling device controls the inverter to switch the excited phases when the applied voltage is less than or equal to a predetermined value that is determined based on the product of the resistance of the coil corresponding to the applied voltage and the current value detected by the first detector.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.